1. Field of the Invention
This invention relates to a method and apparatus for unloading bulk material from a container such as a barge or vessel with a clamshell bucket suspended from a boom which is pivotable through the vertical from a position over the barge to be unloaded to a position over a hopper for receiving the bulk material.
2. Prior Art
It is common practice to unload bulk material from barges or other vessels using clamshell buckets. Generally, these clamshell bucket unloaders are of two types: whirler cranes in which the bucket is suspended from a boom which is slewed in a horizontal plane to transfer the bucket from a position over the vessel to a discharge position over a hopper on shore, and trolley mounted buckets in which the bucket is suspended from a trolley which reciprocates along a fixed horizontal boom cantilevered over the vessel to vertically align the bucket with the hold of the vessel and the hopper on shore. The whirler cranes have a relatively long cycle time which limits their capacity to unload a vessel in a given time period while the trolley mounted bucket unloaders require extensive structure which makes them practical only for relatively large operations. Both the whirler cranes and trolley mounted buckets require playing out the bucket hold ropes if the bucket is to be lowered into the hopper during discharge to reduce "dusting" or pollution of the air with bulk material particles.
It has been suggested in U.S. Pat. No. 3,091,353 that a clamshell bucket can be suspended from a short boom depending from a parallelogram linkage which is operated by a hydraulic ram to place the pivot of the boom over the vessel to be unloaded. The depending boom is then pivoted by another hydraulic ram to reciprocate the bucket from a loading position over the hold to a discharge position over a hopper carried by the parallelogram linkage which feeds an extendable conveyor. Again, this unloader requires a massive structure and the hold ropes must be played out to discharge the bulk material inside rather than above the hopper.
U.S. Pat. No. 1,317,696 suggests an unloader in which a clamshell bucket is supported by a double boom frame which is luffed through the vertical by a pair of gears from a position on one side of the machine wherein the bucket is centered over a railroad car to be unloaded to a position on the other side of the vertical to center the bucket over a wagon in which the bulk material is deposited. When no wagon is available, the bucket can be raised to the vertical and the material deposited in a tilting hopper. In this arrangement the hold and close ropes for the bucket are reeved over sheaves on the axis of rotation of the crane arm such that there is no change in the suspended length of the ropes as the crane passes from the one side of the vertical to the other. For this reason and the fact that the bulk material is discharged into the hopper when the boom is vertical, the ropes must also be played out to empty the bucket inside the hopper and thus reduce dusting.
Other prior art of interest includes U.S. Pat. No. 1,010,291 in which a clamshell bucket is suspended from a boom which is luffed to align the bucket with a hopper; however, the boom is not luffed to or through the vertical and therefore the ropes must be played out to discharge bulk material inside the hopper. U.S. Pat. No. 3,429,453 discloses a crane installation for ships in which a hinged frame straddles the ship's hold and is luffed through the vertical by cables. It is stated that the load carrying hook can be maintained at a constant elevation by suitable controls as the boom is luffed.
It is a primary object of this invention to provide a bulk material unloader which is simple, reliable and has a fast cycle time.
It is another object of the invention to provide such an unloader which through its arrangement and construction automatically lowers the bucket into the discharge hopper to reduce dusting, thereby reducing the load on the operator, minimizing the number of starts of the hoist motor and decreasing cycle time.
Other objects will become clear from the following detailed description of specific embodiments of the invention.